(a) Field of the Invention
The present invention relates to active material for a negative electrode used in lithium-ion batteries, and more particularly, to active material for a negative electrode used in lithium-ion batteries in which the active material made from a crystalline carbon core and an amorphous or turbostratic carbon shell. The present invention also relates to a method of manufacturing such active material.
(b) Description of the Related Art
Carbonaceous material is typically used for the active material in negative electrodes of lithium-ion batteries. There are two basic types of carbonaceous active material for negative electrodes: crystalline carbon and amorphous carbon. Among the different variations of crystalline carbon, graphite is most commonly used, whereas either soft carbon obtained by heat-treating pitch at approximately 1000xc2x0 C. or hard carbon obtained by carbonizing a polymer resin is generally used for the amorphous carbon.
In the case where graphite is utilized in the lithium-ion battery, the manufacture of high voltage batteries is possible since an oxidation reducing potential of this material is low. Further, because of the exceptional voltage flatness of graphite, voltage is evenly discharged, and a high coulomb efficiency of a first cycle is achieved. Here, coulomb efficiency refers to a ratio between an intercalated amount of lithium ions and a deintercalated amount lithium ions on the negative electrode. Since the intercalated and deintercalated amounts are substantially equal in the case of graphite (i.e., the ratio is approximately 1:1), this indicates that a high degree of coulomb efficiency is obtained.
However, a drawback of graphite is that its capacity does not exceed 370 mAh/g (the theoretical capacity of graphite being only 372 mAh/g). As a result, when graphite is used as the negative electrode for a battery, ethylene carbonate generally must be used for the electrolyte. A serious problem of ethylene carbonate is that it becomes a solid at room temperature. When in a solid state, ion conductivity of ethylene carbonate is low, and the material becomes difficult to handle during manufacture, i.e., it is difficult to insert into the battery. In addition, ethylene carbonate is an expensive material. If propylene carbonate, which is less costly than ethylene carbonate and remains a liquid at room temperature, is applied as electrolyte in the battery using graphite as the negative electrode, graphite layers are peeled by co-intercalation of the electrolyte, and, as a result, lithium-ion intercalation is not properly realized. Accordingly, the capacity of the battery is reduced.
To remedy the above problem, U.S. Pat. No. 5,344,726 discloses a method in which a hydrocarbon such as propane undergoes pyrolytic deposition on a surface of graphite to obtain a negative electrode active material in which crystalline carbon is coated with amorphous carbon or carbon having a turbostratic structure. Turbostratic structure here refers to a structure displaying an extremely low level of crystallinity and having a small crystal size such that it is identical to the amorphous structure, and also refers to a structure exhibiting some level of disordered orientation.
However, the processes involved in manufacturing the active material of U.S. Pat. No. 5,344,726 are complicated and difficult to perform. Also, during manufacture, in addition to obtaining the desired negative electrode active material, there is also inadvertently produced negative electrode active material of an amorphous carbon structure. When present in the battery, this negative electrode active material reduces charge and discharge efficiency by the influence of the amorphous carbon. Another significant drawback of this method is that the amorphous carbon or turbostratic structure carbon layer can not be evenly formed on the surface of the crystalline carbon.
In addition to the above method, there have been many attempts to introduce amorphous carbon or carbon of a turbostratic structure on the surface of crystalline carbon. However, such attempts typically result in the surface carbon material being mixed with the core into something approaching a simple mixture such that, rather than improving on the advantages of the carbon core and coating, the disadvantages of both these parts of the active material are magnified.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a high yield active material for a negative electrode in which a surface of a crystalline carbon is evenly coated with an amorphous carbon or a carbon of a turbostratic structure.
It is another object of the present invention to provide active material for a negative electrode in which advantages of a crystalline carbon and an amorphous carbon or a turbostratic structure carbon are fully realized.
It is still another object of the present invention to provide a method for manufacturing active material for a negative electrode used in lithium-ion batteries, and to a method for manufacturing a lithium-ion battery using the active material, in which the methods are simple.
To achieve the above object, the present invention provides active material for a negative electrode used in a lithium-ion battery. The active material includes a crystalline carbon core, and a shell coating the core of an amorphous carbon or a turbostratic structure carbon, a thickness of the shell being between 10 and 2000 xc3x85.
Further, a method of manufacturing the active material includes the steps of chemically combining a crystalline carbon and an amorphous carbon precursor, removing remaining amorphous carbon precursor not reacted in the chemical combination, and heat-treating a chemical combination material of graphite and the amorphous carbon precursor obtained in the chemical combination.
In another aspect, a method of manufacturing the active material includes the steps of introducing a reactive functional group to a surface of a crystalline carbon, chemically combining the reactive functional group to an amorphous carbon precursor, and heat-treating a chemical combination material of the crystalline carbon and the amorphous carbon precursor obtained in the chemical combination.